fb55 fb55 - 25 days ago 19
C Question

Pseudo-generics in C

I need to implement some methods that do stuff with different kinds of number arrays. Usually, I'd use generics for that job, but as C doesn't provide them, I'm now trying to emulate them using macros.

Here's an example of what I'm trying to do:

#ifndef TYPE
#define TYPE int
#endif

TYPE get_minimum_##TYPE (TYPE * nums, int len){
TYPE min = nums[0];

for (int i = 1; i < len; i++) {
if (nums[i] < min) {
min = nums[i];
}
}

return min;
}


However, this won't compile. The clang error message:


error: expected ';' after top level declarator


Is there any way to do this in C? Or do I need implement this for every type by hand?

Answer

You can do something like this in a header file:

//
// generic.h
//

#define TOKENPASTE(x, y) x ## y

#define GET_MINIMUM(T) TOKENPASTE(get_minimum_, T)

TYPE GET_MINIMUM (TYPE) (TYPE * nums, size_t len){
    TYPE min = nums[0];

    for (size_t i = 1; i < len; i++) {
        if (nums[i] < min) {
            min = nums[i];
        }
    }

    return min;
}

and then #include it in a source file for each required type, e.g.:

//
// generic.c
//

#define TYPE int
#include "generic.h"
#undef TYPE

#define TYPE float
#include "generic.h"
#undef TYPE

You can test this by running it through the preprocessor:

$ gcc -E generic.c 

int get_minimum_int (int * nums, size_t len){
    int min = nums[0];

    for (size_t i = 1; i < len; i++) {
        if (nums[i] < min) {
            min = nums[i];
        }
    }

    return min;
}

float get_minimum_float (float * nums, size_t len){
    float min = nums[0];

    for (size_t i = 1; i < len; i++) {
        if (nums[i] < min) {
            min = nums[i];
        }
    }

    return min;
}
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